Advanced Audio Coding Explained

Advanced Audio Coding
Extension:MPEG/3GPP container
  • [[.m4a]], [[.mp4]], [[.3gp]]

Apple container

  • [[.m4a]], [[.m4b]], [[.m4p]], [[.m4r]], [[.m4v]]

ADTS stream

  • [[.aac]]
Noextcode:1
Mime:audio/aac
audio/aacp
audio/3gpp
audio/3gpp2
audio/mp4
audio/mp4a-latm
audio/mpeg4-generic
Released:[1]
Owner:Bell, Fraunhofer, Dolby, Sony, Nokia, LG Electronics, NEC, NTT Docomo, Panasonic
Latest Release Version:ISO/IEC 14496-3:2019
Type:Lossy audio
Contained By:MPEG-4 Part 14, 3GP and 3G2, ISO base media file format and Audio Data Interchange Format (ADIF)
Open:Yes
Free:No[2]
Standard:ISO/IEC 13818-7,
ISO/IEC 14496-3

Advanced Audio Coding (AAC) is an audio coding standard for lossy digital audio compression. It was designed to be the successor of the MP3 format and generally achieves higher sound quality than MP3 at the same bit rate.[3]

AAC has been standardized by ISO and IEC as part of the MPEG-2 and MPEG-4 specifications.[4] [5] Part of AAC, HE-AAC ("AAC+"), is part of MPEG-4 Audio and is adopted into digital radio standards DAB+ and Digital Radio Mondiale, and mobile television standards DVB-H and ATSC-M/H.

AAC supports inclusion of 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 16 low frequency effects (LFE, limited to 120 Hz) channels, up to 16 "coupling" or dialog channels, and up to 16 data streams. The quality for stereo is satisfactory to modest requirements at 96 kbit/s in joint stereo mode; however, hi-fi transparency demands data rates of at least 128 kbit/s (VBR). Tests of MPEG-4 audio have shown that AAC meets the requirements referred to as "transparent" for the ITU at 128 kbit/s for stereo, and 384 kbit/s for 5.1 audio.[6] AAC uses only a modified discrete cosine transform (MDCT) algorithm, giving it higher compression efficiency than MP3, which uses a hybrid coding algorithm that is part MDCT and part FFT.[3]

AAC is the default or standard audio format for iPhone, iPod, iPad, Nintendo DSi, Nintendo 3DS, Apple Music, iTunes, DivX Plus Web Player, PlayStation 4 and various Nokia Series 40 phones. It is supported on a wide range of devices and software such as PlayStation Vita, Wii, digital audio players like Sony Walkman or SanDisk Clip, Android and BlackBerry devices, various in-dash car audio systems, and is also one of the audio formats used on the Spotify web player.[7]

History

Background

The discrete cosine transform (DCT), a type of transform coding for lossy compression, was proposed by Nasir Ahmed in 1972, and developed by Ahmed with T. Natarajan and K. R. Rao in 1973, publishing their results in 1974.[8] This led to the development of the modified discrete cosine transform (MDCT), proposed by J. P. Princen, A. W. Johnson and A. B. Bradley in 1987,[9] following earlier work by Princen and Bradley in 1986.[10] The MP3 audio coding standard introduced in 1994 used a hybrid coding algorithm that is part MDCT and part FFT.[11] AAC uses a purely MDCT algorithm, giving it higher compression efficiency than MP3.[3] Development further advanced when Lars Liljeryd introduced a method that radically shrank the amount of information needed to store the digitized form of a song or speech.[12]

AAC was developed with the cooperation and contributions of companies including Bell Labs, Fraunhofer IIS, Dolby Laboratories, LG Electronics, NEC, Panasonic, Sony Corporation,[13] ETRI, JVC Kenwood, Philips, Microsoft, and NTT.[14] It was officially declared an international standard by the Moving Picture Experts Group in April 1997. It is specified both as Part 7 of the MPEG-2 standard, and Subpart 4 in Part 3 of the MPEG-4 standard.[15]

Standardization

In 1997, AAC was first introduced as MPEG-2 Part 7, formally known as ISO/IEC 13818-7:1997. This part of MPEG-2 was a new part, since MPEG-2 already included MPEG-2 Part 3, formally known as ISO/IEC 13818-3: MPEG-2 BC (Backwards Compatible).[16] [17] Therefore, MPEG-2 Part 7 is also known as MPEG-2 NBC (Non-Backward Compatible), because it is not compatible with the MPEG-1 audio formats (MP1, MP2 and MP3).[18] [19] [20]

MPEG-2 Part 7 defined three profiles: Low-Complexity profile (AAC-LC / LC-AAC), Main profile (AAC Main) and Scalable Sampling Rate profile (AAC-SSR). AAC-LC profile consists of a base format very much like AT&T's Perceptual Audio Coding (PAC) coding format,[21] [22] [23] with the addition of temporal noise shaping (TNS),[24] the Kaiser window (described below), a nonuniform quantizer, and a reworking of the bitstream format to handle up to 16 stereo channels, 16 mono channels, 16 low-frequency effect (LFE) channels and 16 commentary channels in one bitstream. The Main profile adds a set of recursive predictors that are calculated on each tap of the filterbank. The SSR uses a 4-band PQMF filterbank, with four shorter filterbanks following, in order to allow for scalable sampling rates.

In 1999, MPEG-2 Part 7 was updated and included in the MPEG-4 family of standards and became known as MPEG-4 Part 3, MPEG-4 Audio or ISO/IEC 14496-3:1999. This update included several improvements. One of these improvements was the addition of Audio Object Types which are used to allow interoperability with a diverse range of other audio formats such as TwinVQ, CELP, HVXC, speech synthesis and MPEG-4 Structured Audio. Another notable addition in this version of the AAC standard is Perceptual Noise Substitution (PNS). In that regard, the AAC profiles (AAC-LC, AAC Main and AAC-SSR profiles) are combined with perceptual noise substitution and are defined in the MPEG-4 audio standard as Audio Object Types.[25] MPEG-4 Audio Object Types are combined in four MPEG-4 Audio profiles: Main (which includes most of the MPEG-4 Audio Object Types), Scalable (AAC LC, AAC LTP, CELP, HVXC, TwinVQ, Wavetable Synthesis, TTSI), Speech (CELP, HVXC, TTSI) and Low Rate Synthesis (Wavetable Synthesis, TTSI).[26]

The reference software for MPEG-4 Part 3 is specified in MPEG-4 Part 5 and the conformance bit-streams are specified in MPEG-4 Part 4. MPEG-4 Audio remains backward-compatible with MPEG-2 Part 7.[27]

The MPEG-4 Audio Version 2 (ISO/IEC 14496-3:1999/Amd 1:2000) defined new audio object types: the low delay AAC (AAC-LD) object type, bit-sliced arithmetic coding (BSAC) object type, parametric audio coding using harmonic and individual line plus noise and error resilient (ER) versions of object types.[28] [29] [30] It also defined four new audio profiles: High Quality Audio Profile, Low Delay Audio Profile, Natural Audio Profile and Mobile Audio Internetworking Profile.[31]

The HE-AAC Profile (AAC LC with SBR) and AAC Profile (AAC LC) were first standardized in ISO/IEC 14496-3:2001/Amd 1:2003.[32] The HE-AAC v2 Profile (AAC LC with SBR and Parametric Stereo) was first specified in ISO/IEC 14496-3:2005/Amd 2:2006.[33] [34] [35] The Parametric Stereo audio object type used in HE-AAC v2 was first defined in ISO/IEC 14496-3:2001/Amd 2:2004.[36] [37] [38]

The current version of the AAC standard is defined in ISO/IEC 14496-3:2009.[39]

AAC+ v2 is also standardized by ETSI (European Telecommunications Standards Institute) as TS 102005.

The MPEG-4 Part 3 standard also contains other ways of compressing sound. These include lossless compression formats, synthetic audio and low bit-rate compression formats generally used for speech.

AAC's improvements over MP3

Advanced Audio Coding is designed to be the successor of the MPEG-1 Audio Layer 3, known as MP3 format, which was specified by ISO/IEC in 11172-3 (MPEG-1 Audio) and 13818-3 (MPEG-2 Audio).

Blind tests in the late 1990s showed that AAC demonstrated greater sound quality and transparency than MP3 for files coded at the same bit rate.[3]

Improvements include:

Overall, the AAC format allows developers more flexibility to design codecs than MP3 does, and corrects many of the design choices made in the original MPEG-1 audio specification. This increased flexibility often leads to more concurrent encoding strategies and, as a result, to more efficient compression. This is especially true at very low bit rates where the superior stereo coding, pure MDCT, and better transform window sizes leave MP3 unable to compete.

While the MP3 format has near-universal hardware and software support, primarily because MP3 was the format of choice during the crucial first few years of widespread music file-sharing/distribution over the internet, AAC is a strong contender due to some unwavering industry support.[40]

Functionality

AAC is a wideband audio coding algorithm that exploits two primary coding strategies to dramatically reduce the amount of data needed to represent high-quality digital audio:

The actual encoding process consists of the following steps:

The MPEG-4 audio standard does not define a single or small set of highly efficient compression schemes but rather a complex toolbox to perform a wide range of operations from low bit rate speech coding to high-quality audio coding and music synthesis.

AAC encoders can switch dynamically between a single MDCT block of length 1024 points or 8 blocks of 128 points (or between 960 points and 120 points, respectively).

Modular encoding

AAC takes a modular approach to encoding. Depending on the complexity of the bitstream to be encoded, the desired performance and the acceptable output, implementers may create profiles to define which of a specific set of tools they want to use for a particular application.

The MPEG-2 Part 7 standard (Advanced Audio Coding) was first published in 1997 and offers three default profiles:[42]

The MPEG-4 Part 3 standard (MPEG-4 Audio) defined various new compression tools (a.k.a. Audio Object Types) and their usage in brand new profiles. AAC is not used in some of the MPEG-4 Audio profiles. The MPEG-2 Part 7 AAC LC profile, AAC Main profile and AAC SSR profile are combined with Perceptual Noise Substitution and defined in the MPEG-4 Audio standard as Audio Object Types (under the name AAC LC, AAC Main and AAC SSR). These are combined with other Object Types in MPEG-4 Audio profiles.[25] Here is a list of some audio profiles defined in the MPEG-4 standard:[33] [43]

One of many improvements in MPEG-4 Audio is an Object Type called Long Term Prediction (LTP), which is an improvement of the Main profile using a forward predictor with lower computational complexity.

AAC error protection toolkit

Applying error protection enables error correction up to a certain extent. Error correcting codes are usually applied equally to the whole payload. However, since different parts of an AAC payload show different sensitivity to transmission errors, this would not be a very efficient approach.

The AAC payload can be subdivided into parts with different error sensitivities.

Error Resilient (ER) AAC

Error Resilience (ER) techniques can be used to make the coding scheme itself more robust against errors.

For AAC, three custom-tailored methods were developed and defined in MPEG-4 Audio

AAC Low Delay

See main article: AAC-LD.

The audio coding standards MPEG-4 Low Delay (AAC-LD), Enhanced Low Delay (AAC-ELD), and Enhanced Low Delay v2 (AAC-ELDv2) as defined in ISO/IEC 14496-3:2009 and ISO/IEC 14496-3:2009/Amd 3 are designed to combine the advantages of perceptual audio coding with the low delay necessary for two-way communication. They are closely derived from the MPEG-2 Advanced Audio Coding (AAC) format.[45] [46] [47] AAC-ELD is recommended by GSMA as super-wideband voice codec in the IMS Profile for High Definition Video Conference (HDVC) Service.[48]

Licensing and patents

No licenses or payments are required for a user to stream or distribute audio in AAC format.[49] This reason alone might have made AAC a more attractive format to distribute audio than its predecessor MP3, particularly for streaming audio (such as Internet radio) depending on the use case.

However, a patent license is required for all manufacturers or developers of AAC "end-user" codecs.[50] The terms (as disclosed to SEC) uses per-unit pricing. In the case of software, each computer running the software is to be considered a separate "unit".[51]

It used to be common for free and open source software implementations such as FFmpeg and FAAC to only distribute in source code form so as to not "otherwise supply" an AAC codec. However, FFmpeg has since become more lenient on patent matters: the "gyan.dev" builds recommended by the official site now contains its AAC codec, with the FFmpeg legal page stating that patent law conformance is the user's responsibility.[52] (See below under Products that support AAC, Software.) Fortunately, the Fedora Project, a community backed by Red Hat, has imported the "Third-Party Modified Version of the Fraunhofer FDK AAC Codec Library for Android" to its repositories on September 25, 2018,[53] and has enabled FFmpeg's native AAC encoder and decoder for its ffmpeg-free package on January 31, 2023.[54]

The AAC patent holders include Bell Labs, Dolby, ETRI, Fraunhofer, JVC Kenwood, LG Electronics, Microsoft, NEC, NTT (and its subsidiary NTT Docomo), Panasonic, Philips, and Sony Corporation.[14] [13] Based on the list of patents from the SEC terms, the last baseline AAC patent expires in 2028, and the last patent for all AAC extensions mentioned expires in 2031.[55]

Extensions and improvements

Some extensions have been added to the first AAC standard (defined in MPEG-2 Part 7 in 1997):

Container formats

In addition to the MP4, 3GP and other container formats based on ISO base media file format for file storage, AAC audio data was first packaged in a file for the MPEG-2 standard using Audio Data Interchange Format (ADIF),[60] consisting of a single header followed by the raw AAC audio data blocks.[61] However, if the data is to be streamed within an MPEG-2 transport stream, a self-synchronizing format called an Audio Data Transport Stream (ADTS) is used, consisting of a series of frames, each frame having a header followed by the AAC audio data.[60] This file and streaming-based format are defined in MPEG-2 Part 7, but are only considered informative by MPEG-4, so an MPEG-4 decoder does not need to support either format.[60] These containers, as well as a raw AAC stream, may bear the .aac file extension. MPEG-4 Part 3 also defines its own self-synchronizing format called a Low Overhead Audio Stream (LOAS) that encapsulates not only AAC, but any MPEG-4 audio compression scheme such as TwinVQ and ALS. This format is what was defined for use in DVB transport streams when encoders use either SBR or parametric stereo AAC extensions. However, it is restricted to only a single non-multiplexed AAC stream. This format is also referred to as a Low Overhead Audio Transport Multiplex (LATM), which is just an interleaved multiple stream version of a LOAS.[60]

Products that support AAC

HDTV Standards

Japanese ISDB-T

In December 2003, Japan started broadcasting terrestrial DTV ISDB-T standard that implements MPEG-2 video and MPEG-2 AAC audio.In April 2006 Japan started broadcasting the ISDB-T mobile sub-program, called 1seg, that was the first implementation of video H.264/AVC with audio HE-AAC in Terrestrial HDTV broadcasting service on the planet.

International ISDB-Tb

In December 2007, Brazil started broadcasting terrestrial DTV standard called International ISDB-Tb that implements video coding H.264/AVC with audio AAC-LC on main program (single or multi) and video H.264/AVC with audio HE-AACv2 in the 1seg mobile sub-program.

DVB

The ETSI, the standards governing body for the DVB suite, supports AAC, HE-AAC and HE-AAC v2 audio coding in DVB applications since at least 2004.[62] DVB broadcasts which use the H.264 compression for video normally use HE-AAC for audio.

Hardware

iTunes and iPod

In April 2003, Apple brought mainstream attention to AAC by announcing that its iTunes and iPod products would support songs in MPEG-4 AAC format (via a firmware update for older iPods). Customers could download music in a closed-source digital rights management (DRM)-restricted form of 128 kbit/s AAC (see FairPlay) via the iTunes Store or create files without DRM from their own CDs using iTunes. In later years, Apple began offering music videos and movies, which also use AAC for audio encoding.

On May 29, 2007, Apple began selling songs and music videos from participating record labels at higher bitrate (256 kbit/s cVBR) and free of DRM, a format dubbed "iTunes Plus" . These files mostly adhere to the AAC standard and are playable on many non-Apple products but they do include custom iTunes information such as album artwork and a purchase receipt, so as to identify the customer in case the file is leaked out onto peer-to-peer networks. It is possible, however, to remove these custom tags to restore interoperability with players that conform strictly to the AAC specification. As of January 6, 2009, nearly all music on the USA regioned iTunes Store became DRM-free, with the remainder becoming DRM-free by the end of March 2009.[63]

iTunes offers a "Variable Bit Rate" encoding option which encodes AAC tracks in the Constrained Variable Bitrate scheme (a less strict variant of ABR encoding); the underlying QuickTime API does offer a true VBR encoding profile however.[64]

As of September 2009, Apple has added support for HE-AAC (which is fully part of the MP4 standard) only for radio streams, not file playback, and iTunes still lacks support for true VBR encoding.

Other portable players

Mobile phones

For a number of years, many mobile phones from manufacturers such as Nokia, Motorola, Samsung, Sony Ericsson, BenQ-Siemens and Philips have supported AAC playback. The first such phone was the Nokia 5510 released in 2002 which also plays MP3s. However, this phone was a commercial failure and such phones with integrated music players did not gain mainstream popularity until 2005 when the trend of having AAC as well as MP3 support continued. Most new smartphones and music-themed phones support playback of these formats.

Other devices

Supports AAC either by the native Windows Media Player or by third-party products (TCPMP, CorePlayer)

Supports AAC playback in the P-2000 and P-4000 Multimedia/Photo Storage Viewers

plays M4A files containing AAC, and displays metadata created by iTunes. Other Sony products, including the A and E series Network Walkmans, support AAC with firmware updates (released May 2006) while the S series supports it out of the box.

network audio player (made by Slim Devices, a Logitech company) that supports playback of AAC files

supports encoding and decoding of AAC files

supports streaming of AAC through the Zune software, and of supported iPods connected through the USB port

supports AAC files through version 1.1 of the Photo Channel as of December 11, 2007. All AAC profiles and bitrates are supported as long as it is in the .m4a file extension. The 1.1 update removed MP3 compatibility, but according to Nintendo, users who have installed this may freely downgrade to the old version if they wish.[68]

Software

Almost all current computer media players include built-in decoders for AAC, or can utilize a library to decode it. On Microsoft Windows, DirectShow can be used this way with the corresponding filters to enable AAC playback in any DirectShow based player. Mac OS X supports AAC via the QuickTime libraries.

Adobe Flash Player, since version 9 update 3, can also play back AAC streams.[70] [71] Since Flash Player is also a browser plugin, it can play AAC files through a browser as well.

The Rockbox open source firmware (available for multiple portable players) also offers support for AAC to varying degrees, depending on the model of player and the AAC profile.

Optional iPod support (playback of unprotected AAC files) for the Xbox 360 is available as a free download from Xbox Live.[72]

The following is a non-comprehensive list of other software player applications:

a suite of DirectShow and QuickTime plugins which support AAC encoding or AAC/ HE-AAC decoding in any DirectShow application

also supports LC and HE AAC

a free open source DirectShow filter for Microsoft Windows that uses FAAD2 to support AAC decoding

a freeware audio player for Windows that supports LC and HE AAC

an advanced music manager and player that also supports encoding and ripping through a plugin

includes RealNetworks' RealAudio 10 AAC encoder

supports AAC on Windows, Linux and Mac OS X, including the DRM rights management encoding used for purchased music from the iTunes Store, with a plug-in

supports playback and encoding of MP4 and raw AAC files

released with Windows 7, supports playback of AAC files natively

supports AAC (both LC and HE).

supports MP4 playback using a plugin provided by the faad2 librarySome of these players (e.g., foobar2000, Winamp, and VLC) also support the decoding of ADTS (Audio Data Transport Stream) using the SHOUTcast protocol. Plug-ins for Winamp and foobar2000 enable the creation of such streams.

Nero Digital Audio

In May 2006, Nero AG released an AAC encoding tool free of charge, Nero Digital Audio (the AAC codec portion has become Nero AAC Codec),[73] which is capable of encoding LC-AAC, HE-AAC and HE-AAC v2 streams. The tool is a command-line interface tool only. A separate utility is also included to decode to PCM WAV.

Various tools including the foobar2000 audio player and MediaCoder can provide a GUI for this encoder.

FAAC and FAAD2

See main article: FAAC. FAAC and FAAD2 stand for Freeware Advanced Audio Coder and Decoder 2 respectively. FAAC supports audio object types LC, Main and LTP.[74] FAAD2 supports audio object types LC, Main, LTP, SBR and PS.[75] Although FAAD2 is free software, FAAC is not free software.

Fraunhofer FDK AAC

See main article: Fraunhofer FDK AAC. A Fraunhofer-authored open-source encoder/decoder included in Android has been ported to other platforms. FFmpeg’s native AAC encoder does not support HE-AAC and HE-AACv2, but GPL 2.0+ of ffmpeg is not compatible with FDK AAC, hence ffmpeg with libfdk-aac is not redistributable. The QAAC encoder that is using Apple's Core Media Audio is still higher quality than FDK.

FFmpeg and Libav

The native AAC encoder created in FFmpeg's libavcodec, and forked with Libav, was considered experimental and poor. A significant amount of work was done for the 3.0 release of FFmpeg (February 2016) to make its version usable and competitive with the rest of the AAC encoders.[76] Libav has not merged this work and continues to use the older version of the AAC encoder. These encoders are LGPL-licensed open-source and can be built for any platform that the FFmpeg or Libav frameworks can be built.

Both FFmpeg and Libav can use the Fraunhofer FDK AAC library via libfdk-aac, and while the FFmpeg native encoder has become stable and good enough for common use, FDK is still considered the highest quality encoder available for use with FFmpeg.[77] Libav also recommends using FDK AAC if it is available.[78] FFmpeg 4.4 and above can also use the Apple audiotoolbox encoder.[77]

Although the native AAC encoder only produces AAC-LC, ffmpeg's native decoder is able to deal with a wide range of input formats.

See also

External links

Notes and References

  1. Web site: ISO/IEC 13818-7:1997, Information technology — Generic coding of moving pictures and associated audio information — Part 7: Advanced Audio Coding (AAC) . . 1997 . 2010-07-18 . live . https://web.archive.org/web/20120925194258/http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=25040 . 2012-09-25.
  2. Library of Congress . Washington, D.C. . Sustainability of Digital Formats . Full draft . Advanced Audio Coding (MPEG-4) . 22 June 2010 . 1 December 2021.
  3. Web site: MP3 and AAC Explained. Brandenburg. Karlheinz. 1999. dead. https://web.archive.org/web/20170213191747/https://graphics.ethz.ch/teaching/mmcom12/slides/mp3_and_aac_brandenburg.pdf. 2017-02-13.
  4. ISO (2006) ISO/IEC 13818-7:2006 – Information technology — Generic coding of moving pictures and associated audio information — Part 7: Advanced Audio Coding (AAC), Retrieved on 2009-08-06
  5. ISO (2006) ISO/IEC 14496-3:2005 – Information technology — Coding of audio-visual objects — Part 3: Audio, Retrieved on 2009-08-06
  6. Web site: The AAC Audio Coding Family For Broadcast and Cable TV. . 2013. 6. 2024-01-29. https://web.archive.org/web/20230928013141/https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/wp/FraunhoferIIS_White-Paper_AAC-Broadcast-CableTV.pdf. 2023-09-28.
  7. Web site: Audio file formats for Spotify . Spotify . 20 September 2021.
  8. Ahmed . Nasir . N. Ahmed . How I Came Up With the Discrete Cosine Transform . . January 1991 . 1 . 1 . 4–5 . 10.1016/1051-2004(91)90086-Z . 1991DSP.....1....4A .
  9. J. P. Princen, A. W. Johnson und A. B. Bradley: Subband/transform coding using filter bank designs based on time domain aliasing cancellation, IEEE Proc. Intl. Conference on Acoustics, Speech, and Signal Processing (ICASSP), 2161–2164, 1987
  10. John P. Princen, Alan B. Bradley: Analysis/synthesis filter bank design based on time domain aliasing cancellation, IEEE Trans. Acoust. Speech Signal Processing, ASSP-34 (5), 1153–1161, 1986
  11. Web site: Guckert . John . The Use of FFT and MDCT in MP3 Audio Compression . . Spring 2012 . 14 July 2019.
  12. Web site: Borland . John . March 18, 2004 . The sound of science . 2023-04-21 . CNET . en.
  13. Web site: Via Licensing Announces Updated AAC Joint Patent License . . 18 June 2019 . 5 January 2009.
  14. Web site: AAC Licensors . Via Corp . 15 January 2020.
  15. ISO/IEC 14496-3:2009 - Information technology -- Coding of audio-visual objects -- Part 3: Audio . . 1 September 2009 . 2009-10-07 . live . https://web.archive.org/web/20110614010613/http://webstore.iec.ch/preview/info_isoiec14496-3%7Bed4.0%7Den.pdf . 14 June 2011 .
  16. Web site: AAC . MPEG.ORG . 2009-10-28 . https://web.archive.org/web/20091003042614/http://www.mpeg.org/MPEG/audio/aac.html . 3 October 2009 . dead .
  17. Web site: ISO/IEC 13818-7, Fourth edition, Part 7 - Advanced Audio Coding (AAC) . . 15 January 2006 . 2009-10-28 . live . https://web.archive.org/web/20090306055335/http://webstore.iec.ch/preview/info_isoiec13818-7%7Bed4.0%7Den.pdf . 6 March 2009 .
  18. Web site: MPEG-2/MPEG-4 - AAC . 2003 . Gabriel . Bouvigne . MP3'Tech . 2009-10-28 . live . https://web.archive.org/web/20100105022907/http://www.mp3-tech.org/aac.html . 2010-01-05 .
  19. Web site: MPEG Audio FAQ Version 9 - MPEG-1 and MPEG-2 BC . . October 1998 . 2009-10-28 . live . https://web.archive.org/web/20100218081343/http://mpeg.chiariglione.org/faq/mp1-aud/mp1-aud.htm . 2010-02-18 .
  20. Web site: Florence Press Release . . March 1996 . 2009-10-28 . dead . https://web.archive.org/web/20100408061828/http://mpeg.chiariglione.org/meetings/firenze/prfloren.htm . 2010-04-08 .
  21. Johnston, J. D. and Ferreira, A. J., "Sum-difference stereo transform coding", ICASSP '92, March 1992, pp. II-569-572.
  22. Sinha, D. and Johnston, J. D., "Audio compression at low bit rates using a signal adaptive switched filterbank", IEEE ASSP, 1996, pp. 1053-1057.
  23. Johnston, J. D., Sinha, D., Dorward, S. and Quackenbush, S., "AT&T perceptual audio coder (PAC)" in Collected Papers on Digital Audio Bit-Rate Reduction, Gilchrist, N. and Grewin, C. (Ed.), Audio Engineering Society, 1996.
  24. Herre, J. and Johnston, J. D., "Enhancing the performance of perceptual audio coders by using temporal noise shaping", AES 101st Convention, no. preprint 4384, 1996
  25. Web site: MPEG-4 Natural Audio Coding - Audio profiles and levels . Karlheinz . Brandenburg . Oliver . Kunz . Akihiko . Sugiyama . chiariglione.org . 2009-10-06 . dead . https://web.archive.org/web/20100717130019/http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/09-natural_audio_paper/profiles.html . 2010-07-17 .
  26. Web site: ISO/IEC FCD 14496-3 Subpart 1 - Draft - N2203 . ISO/IEC JTC 1/SC 29/WG 11 . 15 May 1998 . 2009-10-07 .
  27. Web site: MPEG-4 Natural Audio Coding - General Audio Coding (AAC based) . Karlheinz . Brandenburg . Oliver . Kunz . Akihiko . Sugiyama . chiariglione.org . 1999 . 2009-10-06 . dead . https://web.archive.org/web/20100219233137/http://mpeg.chiariglione.org/tutorials/papers/icj-mpeg4-si/09-natural_audio_paper/gacoding.html . 2010-02-19 .
  28. Web site: ISO/IEC 14496-3:1999/Amd 1:2000 - Audio extensions . . 2000 . 2009-10-07 . live . https://web.archive.org/web/20110606215234/http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=31568 . 2011-06-06 .
  29. Web site: [ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg4/documents/w2803/w2803_n.pdf ISO/IEC 14496-3:/Amd.1 - Final Committee Draft - MPEG-4 Audio Version 2 ]. ISO/IEC JTC 1/SC 29/WG 11 . July 1999 . 2009-10-07 . http://webarchive.loc.gov/all/20120801161551/ftp://ftp.tnt.uni-hannover.de/pub/MPEG/audio/mpeg4/documents/w2803/w2803_n.pdf . 2012-08-01 . dead .
  30. Web site: MPEG-4 Version 2 Audio Workshop:HILN - Parametric Audio Coding. AES 108th Convention: MPEG-4 Version 2 Audio What is it about?. Paris. Heiko . Purnhagen . 19 February 2000 . 2009-10-07.
  31. Web site: Levels for Audio Profiles . http://webarchive.loc.gov/all/20100108152236/http://www.m4if.org/resources/profiles/audio.php . dead . 2010-01-08 . Fernando . Pereira . MPEG Industry Forum . October 2001 . 2009-10-15 .
  32. Web site: ISO/IEC 14496-3:2001/Amd 1:2003 - Bandwidth extension . . 2003 . 2009-10-07 . live . https://web.archive.org/web/20110606054451/http://www.iso.org/iso/catalogue_detail.htm?csnumber=38148 . 2011-06-06 .
  33. Web site: Text of ISO/IEC 14496-3:2001/FPDAM 4, Audio Lossless Coding (ALS), new audio profiles and BSAC extensions . DOC . ISO/IEC JTC1/SC29/WG11/N7016 . 11 January 2005 . 2009-10-09 . dead . https://web.archive.org/web/20140512215821/http://kikaku.itscj.ipsj.or.jp/sc29/open/29view/29n6475t.doc . 12 May 2014 .
  34. Web site: Audio Lossless Coding (ALS), new audio profiles and BSAC extensions, ISO/IEC 14496-3:2005/Amd 2:2006 . . 2006 . 2009-10-13 . live . https://web.archive.org/web/20120104072435/http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=43026 . 2012-01-04 .
  35. Web site: Audio compression gets better and more complex . Mihir . Mody . Embedded.com . 6 June 2005 . 2009-10-13 . live . https://web.archive.org/web/20160208004827/http://www.embedded.com/design/real-time-and-performance/4025543/Audio-compression-gets-better-and-more-complex . 8 February 2016 .
  36. Web site: MPEG-4 aacPlus - Audio coding for today's digital media world. 2007-01-29 . dead . https://web.archive.org/web/20061026031407/http://www.codingtechnologies.com/products/assets/CT_aacPlus_whitepaper.pdf . 2006-10-26 .
  37. Web site: Parametric coding for high-quality audio, ISO/IEC 14496-3:2001/Amd 2:2004 . . 2004 . 2009-10-13 . live . https://web.archive.org/web/20120104071002/http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=39382 . 2012-01-04 .
  38. Web site: 3GPP TS 26.401 V6.0.0 (2004-09), General Audio Codec audio processing functions; Enhanced aacPlus General Audio Codec; General Description (Release 6) . DOC . 3GPP . 30 September 2004 . 2009-10-13 . live . https://web.archive.org/web/20060819083421/http://www.3gpp.org/ftp/Specs/archive/26_series/26.401/26401-600.zip . 19 August 2006 .
  39. Web site: ISO/IEC 14496-3:2009 - Information technology -- Coding of audio-visual objects -- Part 3: Audio . . 2009 . 2009-10-07 . live . https://web.archive.org/web/20110606214516/http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=53943 . 2011-06-06 .
  40. Web site: AAC . Hydrogenaudio . 2011-01-24 . live . https://web.archive.org/web/20140706172307/http://wiki.hydrogenaud.io/index.php?title=AAC . 2014-07-06 .
  41. US patent application 20070297624 Digital audio encoding
  42. Web site: ISO/IEC 13818-7, Third edition, Part 7 - Advanced Audio Coding (AAC) . https://web.archive.org/web/20110713115817/http://jongyeob.com/moniwiki/pds/upload/13818-7.pdf . dead . 13 July 2011 . . 32 . 15 October 2004 . 2009-10-19 .
  43. 109th AES Convention 2000 September 22–25 Los Angeles . Implementation of MPEG-4 Audio Components on various Platforms . Bernhard . Grill . Stefan . Geyersberger . Johannes . Hilpert . Bodo . Teichmann . Fraunhofer Gesellschaft . July 2004 . 2009-10-09 . dead . https://web.archive.org/web/20070610222853/http://www.iis.fraunhofer.de/fhg/Images/AES5270_MPEG-4_Audio_Components_on_various_Platforms_tcm278-67534.PDF . 2007-06-10 .
  44. Web site: ISO/IEC 14496-3:2009/Amd 3:2012 - Transport of unified speech and audio coding (USAC). ISO. 2016-08-03. live. https://web.archive.org/web/20160308175637/http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=59635. 2016-03-08.
  45. Web site: ISO/IEC 14496-3:2009 - Information technology -- Coding of audio-visual objects -- Part 3: Audio. ISO. 2016-08-02. live. https://web.archive.org/web/20160520232358/http://www.iso.org/iso/catalogue_detail?csnumber=53943. 2016-05-20.
  46. Web site: ISO/IEC 14496-3:2009/Amd 3:2012 - Transport of unified speech and audio coding (USAC). ISO. 2016-08-02. live. https://web.archive.org/web/20160819222620/http://www.iso.org/iso/catalogue_detail.htm?csnumber=59635. 2016-08-19.
  47. Web site: The AAC-ELD Family for High Quality Communication Services MPEG. mpeg.chiariglione.org. 2016-08-02. live. https://web.archive.org/web/20160820105628/http://mpeg.chiariglione.org/standards/mpeg-4/audio/aac-eld-family-high-quality-communication-services. 2016-08-20.
  48. Book: IMS Profile for High Definition Video Conference (HDVC) Service. GSMA. 24 May 2016. 10. live. https://web.archive.org/web/20160818201412/http://www.gsma.com/newsroom/wp-content/uploads//IR.39-v7.0.pdf. 18 August 2016.
  49. Web site: AAC Licensing FAQ Q5 . Via Licensing . 2020-01-15 .
  50. Web site: AAC License Fees . Via Licensing . 2020-01-15 .
  51. Web site: Via Licensing Corporation . AAC PATENT LICENSE AGREEMENT . www.sec.gov . 21 April 2023 . June 5, 2018.
  52. Web site: FFmpeg License and Legal Considerations . ffmpeg.org.
  53. Web site: Commit - rpms/fdk-aac-free - b27d53fbad872ea0ec103653fddaec83238132d9 - src.fedoraproject.org . src.fedoraproject.org.
  54. Web site: Commit - rpms/ffmpeg - 45f894ec0e43a37775393c159021a4ac60170a55 - src.fedoraproject.org . src.fedoraproject.org.
  55. Web site: List of AAC related patents . hydrogenaud.io.
  56. Web site: MPEG Audio FAQ Version 9 - MPEG-4 . D. . Thom . H. . Purnhagen . MPEG Audio Subgroup . chiariglione.org . October 1998 . 2009-10-06 . live . https://web.archive.org/web/20100214205916/http://mpeg.chiariglione.org/faq/mp4-aud/mp4-aud.htm . 2010-02-14 .
  57. Web site: The xHE-AAC Trademark Program. 2021-02-11. Fraunhofer Institute for Integrated Circuits IIS. en.
  58. Web site: Fraunhofer's xHE-AAC Audio Codec Software Extends Native AAC Support In Android P For Better Quality At Low Bitrates. 2020-07-11. Fraunhofer Institute for Integrated Circuits IIS. en.
  59. Web site: ISO/IEC 14496-3:2019. 2022-02-19. ISO. en.
  60. Wolters . Martin . Kristofer . Kjorling . Daniel . Homm . Heiko . Purnhagen . A closer look into MPEG-4 High Efficiency AAC . 3 . 2008-07-31 . dead . https://web.archive.org/web/20031219000444/http://telos-systems.com/techtalk/hosted/HE-AAC.pdf . 2003-12-19 . Presented at the 115th Convention of the Audio Engineering Society, 10–13 October 2003.
  61. Web site: Advanced Audio Coding (MPEG-2), Audio Data Interchange Format . Library of Congress / National Digital Information Infrastructure and Preservation Program . 7 March 2007 . 2008-07-31 . https://web.archive.org/web/20080730205354/http://www.digitalpreservation.gov/formats/fdd/fdd000112.shtml . 30 July 2008 . live.
  62. ETSI TS 101 154 v1.5.1: Specification for the use of Video and Audio Coding in Broadcasting Applications based on the MPEG transport stream
  63. Web site: iTunes Store goes DRM-free. 2009-02-10. Cohen. Peter. 2010-05-27. Macworld. Mac Publishing. https://web.archive.org/web/20090218092311/http://www.macworld.com/article/137946/2009/01/itunestore.html. 18 February 2009 . live.
  64. Web site: Apple AAC. 2021-11-22. Hydrogenaudio. live. https://web.archive.org/web/20211123040120/https://wiki.hydrogenaud.io/index.php?title=Apple_AAC#afconvert. 2021-11-23.
  65. Web site: Gingerbread - Android Developers. Android Developers. 8 May 2018. live. https://web.archive.org/web/20171229061610/https://developer.android.com/about/versions/android-2.3-highlights.html. 29 December 2017.
  66. Web site: Supported media formats - Android Developers. Android Developers. 8 May 2018. live. https://web.archive.org/web/20120311121312/http://developer.android.com/guide/appendix/media-formats.html. 11 March 2012.
  67. Web site: Palm USA. Palm Pre Phone / Features, Details. dead. https://web.archive.org/web/20110524144931/http://www.palm.com/us/products/phones/pre/#techspecs. 2011-05-24 .
  68. Web site: Nintendo - Customer Service - Wii - Photo Channel. nintendo.com. 8 May 2018. live. https://web.archive.org/web/20170505150346/https://www.nintendo.com/consumer/systems/wii/en_na/channelsPhoto.jsp#photo1.1. 5 May 2017.
  69. Web site: Supported Media for Google Cast . 2015-09-22 . live . https://web.archive.org/web/20150923004649/https://developers.google.com/cast/docs/media . 2015-09-23 . | Supported Media for Google Cast
  70. Web site: Statistics - Adobe Flash runtimes. www.adobe.com. 8 May 2018. live. https://web.archive.org/web/20111002084030/http://www.adobe.com/products/player_census/flashplayer/version_penetration.html. 2 October 2011.
  71. Web site: Adobe Delivers Flash Player 9 with H.264 Video Support. 2007-12-04. Adobe press release. 2014-08-20 . dead . https://web.archive.org/web/20140821183435/http://www.adobe.com/aboutadobe/pressroom/pressreleases/200712/120407adobemoviestar.html . 2014-08-21 .
  72. Web site: Xbox.com | System Use - Use an Apple iPod with Xbox 360. https://web.archive.org/web/20070408201732/http://www.xbox.com/en-US/support/systemuse/xbox360/digitalmedia/ipod.htm. dead. April 8, 2007.
  73. Web site: Nero Platinum 2018 Suite - Award-winning all-rounder. Nero AG. 8 May 2018. live. https://web.archive.org/web/20121214011559/http://www.nero.com/eng/downloads-nerodigital-nero-aac-codec.php. 14 December 2012.
  74. Web site: FAAC . AudioCoding.com . 2009-11-03 . live . https://web.archive.org/web/20091211205237/http://www.audiocoding.com/faac.html . 2009-12-11 .
  75. Web site: FAAD2 . AudioCoding.com . 2009-11-03 . live . https://web.archive.org/web/20091211215311/http://www.audiocoding.com/faad2.html . 2009-12-11 .
  76. Web site: December 5th, 2015, The native FFmpeg AAC encoder is now stable!. ffmpeg.org. 26 June 2016. live. https://web.archive.org/web/20160716124052/http://ffmpeg.org/index.html#aac_encoder_stable. 16 July 2016.
  77. Web site: FFmpeg AAC Encoding Guide. Which encoder provides the best quality? ... the likely answer is: libfdk_aac. 11 April 2016. live. https://web.archive.org/web/20160417000546/http://trac.ffmpeg.org/wiki/Encode/AAC#fdk_aac. 17 April 2016.
  78. Web site: Libav Wiki - Encoding AAC. 11 April 2016. dead. https://web.archive.org/web/20160420213129/https://wiki.libav.org/Encoding/aac. 2016-04-20.

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